Lab-on-a-chip (LOC) based device requirements for analyte detection are sensitivity, universality and portability. To this date, these conditions have not been fully met and detection remains the main challenge in the development of LOC technology. Optical detectors, including fluorescence detection, have demonstrated the highest sensitivity. However, optical detectors are not universal and not easily made portable due to the size of the light sources. The use of electrochemical methods is well-suited for integration into portable systems, but they are less sensitive and prone to interferences. From the group of electrochemical sensors, C4D detectors are the most appealing as they fulfill the requirements of portability, universality for charged analytes and acceptable sensitivity.
The principle of C4D in combination with electrophoresis will now be described with reference to FIG. 1. FIG. 1 shows an arrangement of two external metal electrodes 100a, 100b in close proximity to an electrophoretic separation channel 102 in a microfluidic chip 104. The microfluidic chip 104 comprises two polymer sheets, namely top sheet 104a and bottom sheet 104b. The top sheet 104a provides access to reservoirs as will be described below, and the bottom sheet 104b provides the separation channel 102 that has been hot embossed into the bottom sheet 104b. In use, a run buffer reservoir 107, a first sample reservoir 109 and an outlet reservoir 111 of the microfluidic chip 104 are filled with electrophoretic run buffer solution, and a second sample reservoir 113 is filled with target analytes, typically ionic species dissolved in the run buffer solution. A separation voltage is then applied between the second sample reservoir 113 and the first sample reservoir 109. This drives ‘plugs’ of ions 114 into the separation channel 102. Subsequently, the separation voltage is applied between the run buffer reservoir 107 and the outlet reservoir 111 with all other reservoirs floating. This causes the plugs of ions 114 to be driven towards the electrodes 100a, 100b for detection.
The two external metal electrodes 100a, 100b and the electrophoretic separation channel 102 together form the C4D cell or detection cell. When the upstream/emitting electrode 100a emits an AC signal through the channel 102, it is capacitively captured by the downstream/receiving electrode 100b. The electrodes 100a, 100b are in the same plane and are attached to a top plate that seals the channel 102 and are typically placed in an anti-parallel configuration with respect to the length of the channel 102. The applied AC signal (typically 50-600 kHz) from the emitting electrode 100a capacitively couples through the channel 102 to the receiving electrode 100b, resulting in a small current that is amplified by an amplifier 106, rectified and offset-corrected using a rectifier 108, filtered and that undergoes data acquisition using a data acquisition tool (DAQ) 110 and finally recorded in a computing device 112 or other storage device as a DAQ graph.
The C4D cell can be considered as a combination capacitor-resistor-capacitor (CRC) electrical circuit, where the electrodes 100a, 100b and the channel's electric double layer form the capacitors, and the section of the channel 102 between the electrodes 100a, 100b forms the resistor. When a plug of ions 114 is driven through the section of the channel between the electrodes, the measured impedance of the system changes instantaneously because of the change in the resistance due to the different conductivity of the ionic species passing through the electrodes within the background electrolyte. In practical terms, this leads to a sudden change in the zero leveled output voltage or a peak in the DAQ graph. By electrophoresis, separated plugs of ions can be driven through the C4D cell at different times and the corresponding signal recorded, thus obtaining separated peaks according to the times at which the ions cross the C4D cell. Each peak is related by time to a specific ion, and the area under the peaks is related to the concentration of the specific ion. C4D in combination with electrophoresis therefore provides qualitative and quantitative analysis.
The capacitive coupled contactless conductivity detection (C4D) cells reported to date use two electrodes placed externally over the separation channel. An example is illustrated in FIGS. 2(a) and (b), which respectively show a perspective view and plan view of a conventional detection cell. As noted earlier, electrodes 100a, 100b in conventional detectors are fixed to a top plate 200 that seals the separation channel 102 and are typically placed in an anti-parallel configuration with respect to the channel 102. In this configuration, the capacitance coupling to the solution in the channel 102 is inefficient and requires a high frequency and high voltage to couple the signal to detect low concentration samples. High frequencies, however, result in stray capacitance having a more significant effect. Changes in the conductivity of the solution will then only result in a small change over the background signal.
To reduce or eliminate the stray capacitance, different strategies have been employed such as placing a ground plane 202 between the electrodes 100a, 100b to shield their direct crosstalk (as shown in FIGS. 2(a) and (b)). However, while these strategies decrease the stray capacitance somewhat, the resulting detection sensitivity remains limited.
One alternative option to improve capacitance is to increase the magnitude of the AC voltage. However, high voltage levels are difficult to produce and are not safe to handle in portable systems. Another option to have increased capacitance is to use relatively large electrodes or detection lengths, but these approaches severely decrease resolution.
Without compromising resolution, one effective way to increase sensitivity in capacitive coupling detection is to reduce the distance between the electrodes at the detection area or the section of the channel between the electrodes (also known as the ‘detection cell volume’). Known arrangements have achieved this by either: (i) scribing off some portion from the chip surface so that electrodes can be disposed nearer to the channel, or (ii) incorporating electrodes within the chip (integrated chip) during the microfabrication so that they are close enough to the channel. These approaches are either inaccurate (for option (i) above) or require complex fabrication processes (for option (ii) above).
To improve the portability of LOC devices, improvements have been made to releasbly attach the system to a laptop computer. The advantage of portable systems is that tests and analysis can be done onsite and when needed, which improves efficiency and decreases costs. In addition, transportation of the samples exposes the samples to conditions where the sample may possibly be contaminated or degraded. The portable system is user friendly and simple to operate. Therefore, it does not need highly trained personnel to operate the lab-on-a-chip analytical instrument. The system is also low cost and rugged to be carried and transported.
In the past decade, there have been efforts on developing portable instrumentation with somewhat limited success. In the area of electrophoresis, currently a few portable systems have been described where C4D is used as detector. One of them was developed for glass capillaries which typically require higher voltages to substantiate a high electric fields as the capillaries have longer length than microfluidic devices. This system is described in Electroanalysis 19, 2007, No. 19-20, pp. 2059-2065 by Kuban, P; Nguyen, H T A; Macka, M; Haddad, P R; Hauser, P C. Another system is from Innovative Sensor Technologies GmbH called TraceDec. This system uses glass microfluidic chips which are not disposable due to cost. The sensing electrodes are place at a far distance form the defection volume which results on low sensitivity. Another version that has been commercialized is described in IEEE SENSORS JOURNAL, VOL. 8, NO. 5, MAY 2008 by Holger Mühlberger, Wonhee Hwang, Andreas E. Guber, Volker Saile, and Werner Hoffmann. This system uses microchips with detection electrodes that are micro patterned on the outer surface of a plastic microfluidic chip. Due to this extra fabrication step, the cost of these types of chips increases drastically. Another C4D system is being commercialized by eDAQ Pty Ltd This system is modular, not fully integrated and involves the powers supply module, the C4D electronics module and the microfluidic C4D detection platform The electrophoresis microfluidic chip sits atop the C4D platform where the pair of electrodes for transmitting and receiving the AC signal are located only at the bottom plane. The electrodes are brought into contact with the chip from one side only and there is no ground plane between them to shield them from direct coupling to eliminate stray capacitance. The sensing electrodes are of fixed dimensions and not exchangeable. The cell is not encased into a housing to shield the whole cell from external interference effects or noises.
With the present invention, we developed a new generation of portable E-dC4D that provides improved sensitivity, low detection limit and at the same time it is low cost which reduces the price per analysis.
The present invention relates to the use of emitting electrodes positioned or positionable adjacent to and on opposite sides of a microfluidic channel, and receiving electrodes adjacent to and positioned or positionable on opposite sides of a microfluidic channel.
In one specific expression, the present invention relates to a contactless conductivity detection cell including: a microfluidic chip having a channel defined by channel walls, first and second emitting electrodes, and first and second receiving electrodes, wherein the first emitting electrode and the first receiving electrode are adjacent a first channel wall, and the second emitting electrode and the second receiving electrode are adjacent a second channel wall, the second channel wall being opposite the first channel wall.
Preferably the emitting electrodes and receiving electrodes are substantially planar and substantially parallel to each other.
Preferably the emitting electrodes are placed one on top and one at the bottom of the chip passing over the channel, and are configured to act as electrostatic image of each other to concentrate and focus signals from each other into a detection cell volume of the detection cell. Similarly, the receiving electrodes are preferably placed one on top and one at the bottom of the chip covering channel, and are configured to act as electrostatic images of each other to extract coupled signal from a detection cell volume of the detection cell.
Preferably the electrodes are each positioned at a distance of between 1 μm and 1000 μm from the channel and preferably the microfluidic chip has a thickness in the range of 30 μm to 1 mm.
Preferably the detection cell further comprises a first ground plane between the emitting electrodes and the receiving electrodes, and a grounded metal housing containing the emitting electrodes, the receiving electrodes and the first ground plane.
Preferably the detection cell further comprises a second ground plane configured to shield the emitting electrodes and the receiving electrodes from interferences from electronic components housed in the grounded metal housing while keeping a very close distance between the receiving electrodes and the receiving amplifier encased in a second shielded housing.
Preferably at least part of the channel between the emitting electrodes and the receiving electrodes has a restricted submicron-sized or nano-sized width/cross-section.
In one form, the first emitting electrode and the first receiving electrode are preferably arranged on or in a top plate of the microfluidic chip, and where the second emitting electrode and the second receiving electrode are adjacent a base of the channel. In another form, the first emitting electrode and the first receiving electrode are preferably arranged adjacent one side of the channel, and wherein the second emitting electrode and the second receiving electrode are arranged adjacent an opposite side of the channel.
Preferably the detection cell further comprises multiple parallel channels, each channel having a pair of emitting electrodes and a pair of receiving electrodes, wherein all of the emitting electrode pairs are connected to a single input.
In another expression, the present invention relates to a portable electrophoretic micro fluidic system and a contactless conductivity detection system comprising: a platform having an opening configured to receive a microfluidic chip having a channel defined by channel walls, a cover configured to close at least part of the opening, first and second emitting electrodes, and first and second receiving electrodes, wherein the first emitting electrode and the first receiving electrode are configured to be positioned adjacent a first channel wall, and the second emitting electrode and the second receiving electrode are configured to be positioned adjacent a second channel wall, the second channel wall being opposite the first channel wall.
Preferably the system is battery powered, which allows for greater portability.
Preferably the cover is configured to secure at least part of a microfluidic chip between the cover and the base of the opening.
Preferably the second emitting electrode and the second receiving electrode are positioned on or adjacent the base of the opening.
Preferably the first emitting electrode and the first receiving electrode are positioned on or adjacent an internal surface of the cover. Preferably the cover includes a holder on the internal surface, and wherein the first emitting electrode and the first receiving electrode are positioned on the holder.
Preferably the holder is resiliently coupled to the cover and is configured to press the first emitting electrode and the first receiving electrode against a microfluidic chip.
Preferably the detection system further comprises one or more slots to allow a microfluidic chip to be inserted into the opening.
Preferably the cover is selected from a group consisting of: a pivotable cover and a detachable cover.
Preferably the emitting electrodes and/or the receiving electrodes are movable along the channel.
Preferably the opening is configured to allow a microfluidic chip to be movable within the opening.
Preferably the detection system further comprises a current-to-voltage converter adjacent and connected to the receiving electrodes, and a rectifier, low-pass filter, and offset circuit connected to the current-to-voltage converter. Preferably the detection system further comprises an alternating current function generator adjacent and connected to the emitting electrodes, and a miniaturized high voltage power supply. Preferably the detection system further comprises detection electronics arranged on a circuit that comprises a top layer and a bottom layer, the top layer being isolated from the bottom layer.
In yet another specific expression, the present invention relates to a capacitive coupled contactless conductivity detection cell comprising: a microfluidic chip having a channel, and detection electrodes placed in a top-bottom geometry and in close proximity to the channel.
In still another specific embodiment, the present invention relates to a capacitive coupled contactless conductivity detection cell including: detection electrodes placed in a top-bottom geometry in a housing, a detection area located within the housing, a Faraday shield, and a grounded metal housing, wherein the electrodes are shielded from direct cross talk or external noise by the Faraday shield and the grounded metal housing.
Preferably the detection electrodes comprise two emitting electrodes and two receiving electrodes.
Preferably the emitting electrodes are placed one on top and one at the bottom of the channel, and are configured to act as electrostatic images of each other to concentrate and focus the signals from each other into a detection cell volume of the detection cell.
Preferably the receiving electrodes are placed one on top and one at the bottom of the channel, and are configured to act as electrostatic images of each other to extract coupled signals from a detection cell volume of the detection cell.
Preferably the detection electrodes comprise two emitting electrodes and two receiving electrodes separated by the Faraday shield and located in the grounded metal housing.
Preferably the housing holds a microfluidic chip inserted into a holder within the emitting and receiving electrodes.
Preferably the emitting and receiving electrodes are placed close to the separation channel, and are movable by a cover to adjust a detection cell volume of the detection cell.
Preferably the detection area is adjustable by moving the microfluidic chip within the emitting and receiving electrodes.
In one form of the invention the electrodes are provided within a replaceable cartridge cell. In use the cartridge cell is intended to sandwich the detection cell with the microfluidic channel. The cell may be releasably attached to the rest of the detecting system by a mechanism which electrically connects the emitting electrodes and the receiving electrodes to corresponding electrical contacts of the housing. A variety of replaceable cartridges may be provided, suitable respectively for different applications in terms of detection sensitivity and resolution. For example, they may differ in their electrode dimensions and gap distance. Thus, in contrast to embodiments in which these parameters are variable, this form of the invention is robust, and less complex, which reduces cost. A user can adapt the measurement which is made very easily by replacing one cartridge with another.
The present invention provides a C4D cell with improved sensitivity and detection limit compared to detection cells of the state of the art. A benefit of improving sensitivity is that the present invention can be implemented using lower power inputs than previously employed. The gap distance between the detection electrodes is made adjustable in accordance with certain embodiments of the invention, hence the detection cell length and thus the limit of detection (LOD, which determines sensitivity) and/or peak separation (which determines resolution) can be fine-tuned depending on demands of the specific application Where a shielded housing is provided containing all the necessary electronics and the C4D cell, an enhanced signal-to-noise ratio (S/N) is able to be obtained, which results in a highly sensitive portable electrophoretic analyzer. Embodiments of the present invention also provide a C4D detection device that has low power requirements. These and other related advantages will be readily apparent to skilled persons from the description below.